Altered States is a 1980 American science fiction horror film directed by Ken Russell and adapted by playwright and screenwriter Paddy Chayefsky from his 1978 novel of the same name. The novel and the film are based in part on John C. Lilly's sensory deprivation research conducted in isolation tanks under the influence of psychoactive drugs like mescaline, ketamine, and LSD. The film features elements of both psychological horror and body horror.

Chayefsky withdrew from the project after disputes with Russell and took his name off the credits, substituting "Sidney Aaron", his actual first and middle names. The film stars William Hurt, Blair Brown, Bob Balaban, and Charles Haid, and marks the screen acting debuts of Hurt as well as Drew Barrymore (in a small role). The film score was composed by John Corigliano and conducted by Christopher Keene.

Warner Bros. gave the film a limited theatrical release in the United States on December 25, 1980, followed by a wide release in February 1981. The film garnered generally positive reviews from critics, and was nominated for Best Original Score and Best Sound at the 53rd Academy Awards.